This invention relates generally to mobile communications systems and, more specifically, to distributed call setup, admission, control and rerouting in mobile communications systems capable of supporting asynchronous transfer mode.
In recent years, interest has rapidly grown in two distinct areas of the communications field. First, the popularity of mobile, or personal, communications has increased immensely and is expected to grow in the near future to the point where existing systems will be unable to support the demand for mobile communications. The basic problem facing the future of mobile communications systems is the lack of available bandwidth for the mobile user's wireless transmission to a fixed network.
Current mobile communications systems employ the concept of "cells." A cell is a geographical area which is assigned to a corresponding base station which is in turn wired into a fixed communications network. The capacity of a cellular system can become quite high since the available bandwidths can be reused among the various cells. The fixed network is typically a "mesh network." A mesh network is comprised of numerous switches connected together by communication links. The mesh network is set up so that a communications route may be traced from any one switch to any other in the network through at least one, and more often, many combinations of links and switches. Some of the switches in the network, in addition to being connected to other switches, will also be connected via communication links to one or more of the base stations and/or fixed termination points such as a home telephone.
When a mobile user wishes to place a call, the call is transmitted through a communication medium, often a radio channel, to the base station assigned to the user's cell. From the base station, the call is carried by the mesh network to the user's intended destination. When the mobile user moves from one cell to another, a call handoff, or handover, between base stations takes place. The handoffs are performed by a central system controller. This is known as centralized call processing.
Due to the limited bandwidth available for the wireless transmissions of mobile users' calls, each cell can handle only a limited number of callers. Thus, to meet the needs of an increasing population of mobile users, the area assigned to each cell must be decreased. Therefore, the number of such cells, and their corresponding base stations, increases for a given geographical area. The smaller cell size means that the rate of call handoffs per second in a mobile communications system increases, thereby placing a great burden on the central system controller. It is predicted that in the near future, the centralized system controllers of today's systems will be unable to handle the call handoff rate which will be necessary to meet the demand for mobile communications. The need for a distributed call processing system, rather than a centralized system, has been identified, but no comprehensive solutions have as yet been found.
The second area of communications which has been the focus of much research is the standardization of broadband integrated services digital networks (B-ISDN) utilizing the asynchronous transfer mode (ATM). Present communications systems rely on circuit switching techniques. In these circuit switching systems, a circuit path between the caller and his destination is found and used exclusively for a single call until the call terminates. On the other hand, future B-ISDN systems will employ packet, or cell, transport techniques. This means, essentially, that a communication is broken up into discrete "packets", or "cells" (unrelated to the geographic cells of mobile communications systems), which are sent one at a time through the system and received at the intended destination as an uninterrupted communication. Packets from many different callers may simultaneously share the same communication link, thus making these B-ISDN systems a more efficient means of communication than the circuit switching systems.
ATM is the target mode for future B-ISDN systems. The other, and at one time favored, choice for B-ISDN packet communications was synchronous transfer mode (STM) which would handle the packets from a given communication during allocated time slots occurring on a regular basis. For reasons unimportant to this invention, ATM was chosen over STM.
The ATM packets are themselves divided into two sets of information. One set is the information which the user intends to transmit and the other set is called the "header." The header contains routing information, including a virtual channel identifier (VCI). The VCI, simply put, is a code assigned to the packet which lets an ATM switch know where to send the packet next, based on the switch port where the packet has been received.
A conventional ATM B-ISDN switch, hereinafter called an ATM switch, has several input and output ports. Embodied in the implementation of a conventional ATM switch is a "lookup table." The lookup table may be thought of as having four columns: input port, incoming VCI, output port, and outgoing VCI. For every possible input port and incoming VCI combination, there is a corresponding output port and outgoing VCI combination programmed into the lookup table. When an ATM switch receives a packet at a given port, the ATM switch will find the row in the lookup table which has the incoming VCI and input port which correspond to that of the received packet. The ATM switch will then switch or route the packet to the output port which appears in the same row and replace the incoming VCI in the packet header with the outgoing VCI. This process is performed at each switch until the packet arrives at its destination.
The heightened interests in both mobile communications and B-ISDN communication using ATM which has been described above has led to the present invention. Up until now, no system, existing or proposed, has provided a technique for distributed, as opposed to centralized, call setup and rerouting in a mobile ATM based B-ISDN system with several ATM switches.